The present invention relates to an image forming apparatus of an electrophotography system, particularly relates to an image forming apparatus having a transfer apparatus for transferring a toner image onto a transfer member such as a sheet or an intermediate transfer belt.
In the image forming apparatus of the electrophotography system, the toner image is formed onto a photoreceptor and the formed toner image is transferred onto a sheet. Or in the color image forming apparatus in which a full color output is possible, different toner images are formed respectively onto a plurality of photoreceptors and the formed toner images are sequentially superimposed onto an intermediate transfer member. And the superimposed toner image is secondarily transferred onto a sheet collectively in a secondary transfer section.
In such image formation, the toner image is transferred to a sheet or an intermediate transfer member by having the sheet or the intermediate transfer member (these are hereafter called a transfer member) contact the photoreceptor. When separating the transfer member from the photoreceptor after transferring, there may be a case that an unintentional discharge phenomenon between the toner layers on the photoreceptor and a transfer member may occur. Since the charge of some toners changes by the abnormal discharge when such abnormal discharge occurs, the electrostatic absorption force with the transfer member will decline. Under this effect, the spread of the toner image arises by opposing with a surrounding toner, or a reverse transfer is carried out to the other photoreceptor on the downstream side. These phenomena cause image failures.
FIG. 9 illustrates a schematic diagram explaining the image failure by the abnormal discharge in a tandem type color image forming apparatus. The figure shows the circumference of the intermediate transfer belt 6 of the shape of a belt in a color image forming apparatus. The toner images, each of which has different color, are respectively formed onto a plurality of drum shaped photoreceptors 1. The formed toner image is transferred onto intermediate transfer belt 6 by a transfer roller 7, and is sequentially superimposed.
In the superimposed toner layer, since the total charge amount of the toner and the thickness of the toner layer increase compared to a monolayer toner layer, the absolute value of the toner layer potential rises. In the toner layer, which has been superimposed on the 2nd photoreceptor 1 (M), an exfoliation discharge phenomenon between the toner layer and the photoreceptor 1 by the rise of the toner layer potential is easily generated. When an exfoliation discharge “ed” occurs as shown in the figure, in connection with the discharge phenomenon, the charge amount will change in some toners of the toner layer, and the toner, which has been originally tinged with a negative charge, becomes a toner “pt” having a positive charge depending on the change level. In a photoreceptor 1 (C) located in further downstream, the toner “pt” of the positive charge is reversely transferred “rd” to the photoreceptor 1 (C) by a transfer electric field in a transfer nip N. For this reason, in a portion where the reverse transfer has generated, the amount of the toners will decrease compared to the surrounding, and an image failure will occur.
In order to suppress the abnormal discharge, Unexamined Japanese Patent Application Publication No. 2001-154548 discloses an image forming apparatus having a cleaning discharger, which is arranged to apply alternative bias voltage onto the photoreceptor so as to discharge a part of electric charges generated by the cleaning discharger towards the surface of the transfer member, onto which a toner image has been transferred. Further, Unexamined Japanese Patent Application Publication No. 2005-115197 discloses an image forming apparatus with a discharger which discharges a front surface or a back surface of the intermediate transfer member in the downstream of the transfer position.
According to the image forming apparatus disclosed by Unexamined Japanese Patent Application Publication Nos. 2001-154548 and 2005-115197, in either case, a discharge electrode actively self-discharges to the transfer member to control the electric charge of the toner layer formed on the transfer member in a prescribed range.
However, it is difficult to control the electric charge of the toner layer in the prescribed range. When not charged uniformly, it becomes an uneven charge of the toner layer, and the spread of the toner image resulting from the uneven charge and the image failure by the reverse transfer on the photoreceptor drum on the downstream side will occur.
Unexamined Japanese Patent Application Publication Nos. 2001-154548 and 2005-115197 do not take the potential of the tone layer into consideration. When the absolute value of the toner layer potential is high, the image failure by the abnormal discharge mentioned above tends to occur. In the image forming apparatus disclosed by the above-mentioned patent documents, since discharge is performed uniformly, unnecessary discharge is performed without taking the toner layer potential into consideration, even when the absolute value of toner layer potential is low where the image failure is not generated.
When performing the discharge in case where the absolute value of the toner layer potential is low, it easily becomes a superfluous discharge. When it becomes the superfluous discharge, the image failure resulting from the uneven charge will occur. Thus, when the discharge was uniformly performed regardless of the level of the toner layer potential, problems, such as a new image failure by the unnecessary discharge and a deterioration of the endurance of the discharge electrode and a loss of consumption energy, may arise.